Joule Thief Battery Charger





Introduction: Joule Thief Battery Charger

About: Why fix it if it ain't broken? Because it's fun.

Many of you are probably aware of the "joule thief" circuit. If you aren't, it's a very simple voltage booster that is normally used to power an LED off of a mostly dead AA battery, but it can be used for other applications where a voltage boost is needed. One such application is charging a battery off of a battery with a smaller volatge rating. There are designs out there for battery chargers using a 9 volt battery and then step down the voltage to the level of the battery to be charged. But this circuit uses any single battery with a rating of 1.5 volts, such as a AA, C, or D battery, and bumps the voltage up to a level that can charge a battery with a voltage rating of 5 volts or lower. This curcuit is extremely simple, dirt cheap, and can be built in an afternoon using parts taken from other old devices. 

Step 1: Parts and Pieces

Parts for the charger include:
2N3904 NPN transistor
1K ohm resistor
Toroid transformer core
Magnet wire
SPST Switch (not pictured)

A word about some of the parts:
The toroid core can be any size, but nothing fancy or big is needed. A small toroid is probably better just because it is more compact. Only one toroid is used in the circuit, but two are pictured for size reference. 
The diode can be just about any diode, but the lower the forward voltage drop, the better. Germanium diodes work the best. To find the forward voltage drop, simply hook a diode up to a battery, and measure the difference in voltage with and without the diode in the circuit. If the voltage drop is about equal to the voltage being supplied, make sure the diode isn't hooked up backwards.

Step 2: Make the Transformer

The toroid transformer needs to be wound first. Cut two pieces of magnet wire to roughly equal length. About three feet each is plenty for a small toroid. Next, wind them parellel to eachother around the toroid transformer core, making sure at no point the wires cross. glue can be used to secure the wires to the core so the transformer does not unwind. The number of coils is not a big deal, as long as each wire has an equal number of coils, and each wire is wound around 10 or more times. Keep winding all the way around the core, and when you are done you should have four loose wire ends coming off the core.

Step 3: Schematic

Now that the transformer has been prepared, its time to look at the schematic and begin assembling the circuit. The schematic is very simple. Its obvious which components are which, but just incase:
B1 is the AA, C, or D battery cell powering the circuit
S1 is a SPST switch
T1 is the toroid transformer
R1 is the 1K ohm resistor
Q1 is the 2N3904 transistor
D1 is the diode
the voltage out connection goes to the battery to be charged.

Step 4: Assmble the Circuit

I chose to make my charger as small as possible, and designed to use any 1.5 volt battery as a power supply, and then be able to charge any other battery. This means that i did not use any battery holders, but if there is a specific type of battery you want to chrge or use as a power supply, you are welcome to use a battery holder. I also removed the switch from the circuit. If you want to make it small like I did, just refer to the pictures for how to connect the parts.

When connecting the transformer, you must pay attention to the phase of the windings. The dots on the schematic represent the phasing. I'm not going to go over the general idea of phasing, just give the specifics to this transformer. On the reansformer, you have coil A and coil B, and side 1 on the left and side 2 on the right. Each end of the coil should be on the opposite side of the transformer from the other. i.e., one end of A is on side 1 (A1) and one end of A should be on side 2 (A2) and the same is with coil B. Choose two ends, one from each coil and from opposite sides on the transfromer, and connect them. Putting it simply, connect A1 and B2 OR B1 and A2. 
This forms the positive connection to the circuit. But which of the two ends of the transformer is connected to what part of the circuit does not matter for the rest of the construction. 

Step 5: Finishing Touches

You can mount your circuit in an enclosure, or however suits your needs. I wanted my circuit to be flexible in terms of what battery is used as a power supply, so i mounted mine on a popsicle stick (very high tech) with a magnet to hold the popsicle stick to the power supply battery. A springy wire extends down to make contact with the positive battery terminal, and a wire with an attatched magent makes contact with the negative terminal. This allows the circuit to be mounted on a battery of any size. The voltage output terminals are just two wire with alligator clips on the end, so the circuit can be hooked up to a battery holder for the battery that is charging or hooked onto the battery by some other means.

Step 6: Use It!

The circuit can now be used to charge batteries from a battery. This might seem silly, until you realize any batter up to 5 volts can be charged off a AA, C, or D battery. I have charged my ipod (almost fully) off a C battery, and my phone (up tp 1/4 full) off of a D, but this can also be used to charge batteries for other devices. the circuit might not be the most efficent boost-type charger circuit, but it uses very common parts and is great when you lose power and dot have any other options. Not only does it act as a charger, but a LED can be powered off of it like a traditional joule thief. (as a side note; I have found this circuit to not be compatible with certain cell phones, even when the battery is removed and charged seperately. Some cell phone batteries have protection circuits built in, and will only be charged while in the phone. Using this on a battery with a protection circuit built in could lead to the damage of that circuit. If the battery pack has more than 2 wires or contacts, then it probably has a protection circuit. aways test on an extra battery)

Congratulations on the completion of your new joule thief battery charger!

With regards to the epilog laser challenge:

I do fabrication of miniature, portable, battery powered guitar amps for many of my friends who are in bands. When I was building one, someone questioned if they could have custom engraving on the front aluminum panel. Unfortunately, I do not have any means to engrave the panel, but if I win the laser engraver  it could solve that! I also do custom woodwork to gun stocks, and being able to laser engrave stocks would really expand my capabilities for customization. 



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    1 Questions


    Does this circuit output AC or DC current to the charging batteries?


    Pulsating DC by virtue of the one way current flow diode D1


    zener diode or we can use general diode

    A cat-whisker diode. The reason it is being used is that it has a very low voltage drop. Many of these diodes dont have numbers printed on them unfortunately. Old audio, radio, or television equipment is a good source of these.

    I would be interested to see you measure the charging current here

    10 replies

    If i am reading my meter right, i am getting 13ma for a AA, 15ma for a C, and 18ma for a D battery. This is with the meter hooked directly into the voltage out terminals.

    cool! could you post a video on youtube? and whats ther current draw fro the battery? i was thinking about using a tree battery and this circuit to make tree lights :D

    Im sorry, but whats a tree battery? and when you refer to the current draw, are you talking about the charger drawing out of the battery? Cause the output current without a load is actually listed in the comment above yours.

    a tree battery was something i found on the internet. pound a few pennies into a tree and parallel them, and parallel galvanized nails a foot lower on the tree. i get about .3 volts off of it, and i thought if i put some in series, i could make tree powered walkway lights! lol that would be just hilarious!

    Hi iamdarkyoshi, I'm wondering if you can give me more details on how to make a tree battery. I looked on line but didn't find any thing quite like you made. It would be good to get 3 v from a tree. Also, does the tree run down?


    That sounds pretty cool. It would be interesting to see what kind of voltages can be acheived using a joule thief running off a tree battery. I dont know that it would be enough though to power a whole walkway though. Maybe use the tree batteery power to charge some batteries that then power the walkway lights

    thts what i was gonna do. but... maybe one joule theif to get tree up to 1.5 volt for battery, and another to get it to 4.5. my walkways are really weird anyway, and ther not very long O.o

    I've tried running one joule thief of another, but t doesnt work directly. What you need to do is have the first one charge a battery that the second one runs off of.

    Provided you keep the voltage above the charging batteries nominal voltage and 13mA is constant, how many dead AA's would it take to charge another AA battery? For a 2000mAH AA battery with 200mA charge current it will take 12hrs - 14hrs to fully charge it. At 13mA charge current it will take you roughly 15X as long
    and you will need a hand full of batteries to do it as well.

    The circuit works nice for a White LED power source, but I would rather call the device in this configuration more of a Trickle Charger, if that. Do you have any data on how long it takes to charge a particular battery and how many dead AA's it takes to do the job?

    I'm sure you've found an answer by now, but if not/anyone else reading this gets curious:

    The output voltage is dependent on the voltage needed to make the magnetic field dissipate. This is a "self regulating" boost converter, which essentially makes a magnetic field then turns itself off, leaving nowhere for that field to go. As it tries to equalize, it will force itself through the breakdown voltage of the transistor (typically 30-50v). However, if you have a few LEDs that only need 10v to run, it will take that magnetic field and turn it into 10V (with the equivalent lesser current running through the LEDs). This will happen ~50k times a second.

    So effectively, this takes .7+ volts (Vi) at Ii amps to be <(reverse breakdown voltage) (Vo) at (Vi*Ii)/(Vo)*efficiency (40-90%).

    humm.. Thanks to your answer.. so now I know.
    Now I will make my own design for portable charger..
    so coming soon from now.... hehehe :)